Coated lithium-nickel composite oxide particles, and method for producing coated lithium-nickel composite oxide particles

ABSTRACT

Provided are excellent coated lithium-nickel composite oxide particles with which it is possible, due to the high environmental stability thereof, to minimize the incidence of impurities owing to absorption of moisture and carbon dioxide gas, said particles having high adhesiveness such that the coating layer does not easily delaminate, and having lithium-ion conductivity. The coated lithium-nickel composite oxide particles, in which an electroconductive polymer is cross-linked to the lithium-nickel composite oxide particles by a three-dimensional structure, are electrically and ionically conductive, and the compound is capable of suppressing the transmission of moisture and carbon dioxide. It is therefore possible to provide coated lithium-nickel composite oxide particles for a lithium-ion cell positive-electrode active substance that is excellent for use in a lithium-ion cell.

TECHNICAL FIELD

The present invention relates to coated lithium-nickel composite oxideparticles with a high content of nickel, and also relates to coatedlithium-nickel composite oxide particles of which the stability underthe atmosphere is improved and which is easy to handle, and a method forproducing the coated lithium-nickel composite oxide particles.

BACKGROUND ART

In recent years, along with the rapid expansion of small-sizedelectronic devices such as cellular phones and laptop computers, ademand for a lithium-ion secondary battery as a chargeable anddischargeable power source has been rapidly increased. A lithium-cobaltoxide (hereinafter, sometimes also referred to as cobalt-based) has beenwidely used as a positive-electrode active substance contributing to thecharging and discharging in a positive electrode of a lithium-ionsecondary battery. However, capacity of the cobalt-based positiveelectrode has improved to the extent of theoretical capacity through theoptimization of battery design, and higher capacity is becomingdifficult to achieve.

Accordingly, lithium-nickel composite oxide particles using alithium-nickel oxide that has the theoretical capacity higher than thatof the conventional cobalt-based one has been developed. However, thepure lithium-nickel oxide has a problem in terms of safety, cyclecharacteristics, and the like because of the high reactivity with water,carbon dioxide, or the like, and is difficult to be used as a practicalbattery. Therefore, lithium-nickel composite oxide particles to which atransition metal element such as cobalt, manganese, and iron, oraluminum has been developed as an improvement measure for the problemdescribed above.

In the lithium-nickel composite oxide, there are composite oxideparticles expressed by a transition metal composition ofNi_(0.33)Co_(0.53)Mn_(0.33), a so-called ternary composite oxide(hereinafter, sometimes referred to as ternary), which is made by addingnickel, manganese, and cobalt in an equimolar amount, respectively, andlithium-nickel composite oxide particles with a nickel content exceeding0.65 mol, a so-called nickel-based composite oxide (hereinafter,sometimes referred to as nickel-based). From the viewpoint of capacity,a nickel-based with a large nickel content has a great advantage ascompared to a ternary.

However, the nickel-based is characterized by being more sensitivedepending on the environment as compared to a cobalt-based or a ternary,because of the high reactivity with water, carbon dioxide, and the like,and absorbing moisture and carbon dioxide (CO₂) in the air more easily.It has been reported that the moisture and carbon dioxide are depositedon particle surfaces as impurities such as lithium hydroxide (LiOH), andlithium carbonate (L₂CO₃), respectively, and have an adverse effect onthe production process of a positive electrode or battery performance.

By the way, the production process of a positive electrode passesthrough a process in which a positive electrode mixture slurry obtainedby mixing lithium-nickel composite oxide particles, a conductiveauxiliary, a binder, an organic solvent, and the like is applied onto acollector made of aluminum or the like, and dried. In general, in theproduction process of a positive electrode mixture slurry, lithiumhydroxide causes the slurry viscosity to increase rapidly by reactingwith a binder, and may cause gelation of the slurry. These phenomenacause faults and defects, and a decrease of production yield of apositive electrode, and may cause a variation in quality of theproducts. Further, during charging and discharging, these impuritiesreact with an electrolytic solution and sometimes generate gas, and maycause a problem in the stability of the battery.

Accordingly, in a case where a nickel-based is used as apositive-electrode active substance, in order to prevent the generationof impurities such as the above-described lithium hydroxide (LiOH), theproduction process of a positive electrode is required to be performedin a dry (low humidity) environment in a decarbonated atmosphere.Therefore, there is a problem that in spite of having high theoreticalcapacity and showing great promise as a material of a lithium-ionsecondary battery, the nickel-based requires high cost for theintroduction of a facility and high running costs for the facility inorder to maintain the production environment, and which becomes abarrier to it becoming widespread.

In order to solve the problem described above, a method of coatingsurfaces of lithium-nickel composite oxide particles by using a coatingagent has been proposed. Such a coating agent is roughly classified asan inorganic coating agent and an organic coating agent. As theinorganic coating agent, a material such as titanium oxide, aluminumoxide, aluminum phosphate, cobalt phosphate, and lithium fluoride havebeen proposed, and as the organic coating agent, a material such asfumed silica, carboxymethyl cellulose, and a fluorine-containing polymerhave been proposed.

For example, in Patent Document 1, a method of forming a lithiumfluoride (LiF) or fluorine-containing polymer layer on surfaces oflithium-nickel composite oxide particles has been proposed, and inPatent Document 2, a method of forming a fluorine-containing polymerlayer onto lithium-nickel composite oxide particles, and further addinga Lewis acid compound to neutralize impurities has been proposed. In anyprocessing, lithium-nickel composite oxide particles are modified so asto have the hydrophobic property with a coated layer containing afluorine-based material, and the adsorption of moisture is suppressed,and the deposition of impurities such as lithium hydroxide (LiOH) can besuppressed.

However, the coated layer containing a fluorine-based material used forthe coating does not have electrical conductivity. Accordingly, eventhough the deposition of the impurities can be suppressed, the coatedlayer itself becomes an insulator, and causes an increase in positiveelectrode resistance and a decrease in battery characteristics.Therefore, there has been a problem in that the quality of thelithium-nickel composite oxide particles itself is lowered.

Patent Document 1: Japanese Unexamined Patent Application, PublicationNo. 2013-179063

Patent Document 2: Japanese Unexamined Patent Application (Translationof PCT Application), Publication No. 2011-511402

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In view of the above-described problems of conventional technique, anobject of the present invention is to provide coated lithium-nickelcomposite oxide particles that can be handled in the atmosphere and canobtain coated films of a lithium ion conductor, the coated films nothaving an adverse effect on battery characteristics, and a method forproducing the coated lithium-nickel composite oxide particles.

Means for Solving the Problems

As a result of intensive studies to solve the above-described problemsof conventional technique, the present inventors have found that bycoating surfaces of nickel-based lithium-nickel composite oxideparticles with a conductive polymer having both electrical conductivityand ion conductivity, the increase of the positive electrode resistanceand a decrease in the battery characteristics due to the coating can beprevented. Further, as for the coated lithium-nickel composite oxideparticles, the coated layer does not peel off from the particle surfaceseven when a positive electrode mixture slurry is kneaded. Furthermore,the present inventors have found suitable coated lithium-nickelcomposite oxide particles that can suppress the generation of impuritiescaused by moisture and carbon dioxide in the air, and can be handled inthe atmosphere during the handing of materials, during transportation,during storage, during the preparation of electrodes and the productionof batteries, and a method for producing the coated lithium-nickelcomposite oxide particles; and thus have completed the presentinvention.

That is, a first aspect of the present invention is coatedlithium-nickel composite oxide particles for a lithium-ion batterypositive-electrode active substance, including nickel-basedlithium-nickel composite oxide particles, surfaces of which are coatedwith a conductive polymer.

A second aspect is the coated lithium-nickel composite oxide particlesaccording to the first aspect of the invention, in which a coatingamount of the conductive polymer is from 0.1 to 5.0% by mass based on100% by mass of the lithium-nickel composite oxide.

A third aspect is the coated lithium-nickel composite oxide particlesaccording to the first or second aspect of the invention, in which theconductive polymer is a polymer or copolymer including at least oneselected from the group consisting of polypyrrole, polyaniline,polythiophene, poly(p-phenylene), polyfluorene, and a derivativethereof.

A fourth aspect is the coated lithium-nickel composite oxide particlesaccording to any one of the first to third aspects of the invention, inwhich the lithium-nickel composite oxide is represented by the followingFormula (1),

Li_(x)Ni(_(l-y-z))M_(y)N_(z)O₂  (1)

(in the formula, x is a value of from 0.80 to 1.10, y is a value of from0.01 to 0.20, z is a value of from 0.01 to 0.15, and l-y-z is a valueexceeding 0.65, and M represents at least one element selected from Coor Mn, and N represents at least one element selected from Al, In orSn.)

A fifth aspect is a composite oxide positive-electrode active substancefor lithium-ion batteries according to any one of the first to fourthaspects of the invention, in which the coated lithium-nickel compositeoxide particles are spherical particles having an average particlediameter of from 5 to 20 μm.

A sixth aspect is a method for producing the coated lithium-nickelcomposite oxide particles according to any one of the first to fifthaspects of the invention, including: preparing a resin solution forcoating by dissolving a conductive polymer into a good solvent thatdissolves the resin for coating; adding a poor solvent that does notdissolve a resin for coating and has a boiling point higher than that ofthe good solvent into the resin solution for coating; adding thelithium-nickel composite oxide into the resin solution for coating toprepare a slurry; and removing the good solvent and the poor solventsequentially from the slurry.

Effects of the Invention

The present invention is excellent coated lithium-nickel composite oxideparticles which have favorable electrical conductivity and lithium ionconductivity on surfaces of lithium-nickel composite oxide particles andare coated with films that can suppress the permeation of moisture andcarbon dioxide by producing coated lithium-nickel composite oxideparticles having a core of nickel-based lithium-nickel composite oxideparticles and a shell including a conductive polymer, and a method forproducing the coated lithium-nickel composite oxide particles.

The coated lithium-nickel composite oxide particles can be provided as ahigh capacity composite oxide positive-electrode active substance for alithium-ion battery, for which production equipment that has been usedfor a cobalt-based and ternary can also be used instead ofpositive-electrode production equipment in which carbon dioxideconcentration and moisture concentration are strictly controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a change rate per particles mass in a case after standingfor one week in Examples and Comparative Example.

FIG. 2 shows a capacity change rate from a cycle test in Examples andComparative Example.

FIG. 3 shows a Cole-Cole plot from an impedance test before the cycletest.

FIG. 4 shows a Cole-Cole plot from an impedance test after the cycletest of 500 cycles.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Hereinafter, coated lithium-nickel composite oxide particles and amethod for producing the same, according to the present invention, willbe described in detail. However, the present invention should not beconstrued as being limited to the following detailed explanation. In thepresent invention, there may be a case where a secondary particleaggregated with primary particles is referred to as lithium-nickelcomposite oxide particles.

The conductive polymer coating the particle surfaces have favorableelectrical conductivity and ion conductivity, therefore, it does notexert an adverse effect on the battery characteristics. Further, thecoated lithium-nickel composite oxide particles coated with a conductivepolymer are excellent in terms of environmental stability because theconductive polymer serves as the coated layer, and can be handled in asimilar facility to that of the cobalt-based or the ternary.Accordingly, the present invention is excellent coated lithium-nickelcomposite oxide particles having conductivity and environmentalstability.

[Conductive Polymer]

The conductive polymer coating the lithium-nickel composite oxideparticles according to the present invention is the designation of ahigh molecular compound having electrical conductivity. This highmolecular compound is characterized by having a structure in which adouble bond and a single bond are alternately arranged in the molecularstructure, that is, having a main chain with developed π conjugation. Ingeneral, in addition to the conductive polymer, by doping an acceptormolecule called a dopant or a donor molecule, a carrier is generated,and electrical conductivity is developed. Examples of the dopantinclude, for example, an alkali metal ion such as Li⁺, Na⁺, K⁺, and Cs⁺,an alkyl ammonium ion such as tetraethylammonium, halogens, Lewis acid,protons, and a transition metal halide.

The conductive polymer is a polymer in which π conjugation is highlydeveloped as represented by polyacetylene, and does not dissolve in anysolvent, does not have a melting point, and has a so-called insolubleand infusible nature. Therefore, the processability is poor andindustrial applications have been difficult.

However, according to a recent study, a conductive polymer obtainedsubstantially or apparently as a solution has been developed, as bydissolving the conductive polymer into an organic solvent, by dispersingthe conductive polymer into a water solvent, or the like, and thus hasbeen widely used for industrial purposes.

Hereinafter, the present invention will be described in detail byexamples. The first example is a method of providing organic solventsolubility or water solubility by directly introducing a substituentinto a monomer constituting a conductive polymer. When describedspecifically, it is known that a polythiophene derivative synthesizedfrom poly-3-alkyl-substituted thiophene to which an alkyl group has beenintroduced at the 3-position of thiophene is dissolved in an organicsolvent such as chloroform, and methylene chloride, and has a meltingpoint before decomposition, that is, is melted and dissolved. Further, apolythiophene derivative synthesized from poly-3-alkyl sulfonic acidthiophene to which alkyl sulfonic acid has been introduced at the3-position obtains water solubility by a sulfo group that is easy to beblended with water, and can result in self doping at the same time.

Further, the second example is a method of using a water-soluble dopant.By introducing a polymer having a sulfo group that is easy to be blendedwith water in the molecule together with a dopant and water dispersantthe conductive polymer can be finely dispersed in water. Whenspecifically described, a monomer constituting the conductive polymer issubjected to oxidative polymerization in an aqueous solution of awater-soluble polymer. At this time, a conductive polymer is doped withpart of the sulfo group having a water-soluble polymer, and further thewater-soluble polymer and the conductive polymer are integrated witheach other, and a water-soluble conductive polymer is obtained by theremaining sulfo groups. The conductive polymer can be finely dispersedin water at a level of several tens of nm. The representative example isPEDOT/PSS developed by using polystyrene sulfonic acid (PSS), and using3,4-ethylene dioxythiophene (EDOT) for the conductive polymer monomer.

For example, a polypyrrole-based compound, a polyaniline-based compound,a polythiophene compound, a poly(p-phenylene) compound, a polyfluorenecompound, or a derivative thereof can be included as the high molecularcompound that can be used in the present invention. Because the presentinvention passes through a process of dissolving or dispersing aconductive polymer into a solvent, for example, lignin graft typepolyaniline or the like in which PEDOT/PSS or lignin has been modifiedat the end of the polyaniline, and which is enriched in the solubilityor the dispersibility, can be preferably used.

Further, the coating amount of the conductive polymer is preferably from0.1 to 5.0% by mass, and more preferably from 0.2 to 0.5% by mass basedon 100% by mass of the nickel-based lithium-nickel composite oxideparticles. When the coating amount is less than 0.1% by mass, theprocessing tends to be insufficient, and when the coating amount exceeds5.0% by mass, the packing density of particles is lowered by aconductive polymer that is not involved in the particles coating, and anadverse effect may be exerted during the production of positiveelectrode.

[Nickel-based Lithium-nickel Composite Oxide Particles]

The nickel-based lithium-nickel composite oxide particles are sphericalparticles, and have the average particle diameter preferably of from 5to 20 μm. When the average particle diameter is set in the range,favorable battery performance is provided as the lithium-nickelcomposite oxide particles, and further favorable battery repetition life(cycle characteristics) is also provided, both can be achieved,therefore, this is preferred.

In addition, the nickel-based lithium-nickel composite oxide particlesare preferably represented by the following Formula (1).

Li_(x)Ni_((l-y-z))M_(y)N_(z)O₂  (1)

in the formula, x is a value of from 0.80 to 1.10, y is a value of from0.01 to 0.20, z is a value of from 0.01 to 0.15, and l-y-z is a valueexceeding 0.65, and M represents at least one element selected from Coor bin, and N represents at least one element selected from Al, In orSn.

Further, the value of l-y-z (nickel content) is, from the viewpoint ofthe capacity, preferably a value exceeding 0.70, and more preferably avalue exceeding 0.80.

The cobalt-based (LCO), the ternary (NCM), and the nickel-based (NCA)have an electrode energy density (Wh/L) of 2160 Wh/L (LiCoO₂), 2018.6Wh/L (LiNi_(0.33)Co_(0.33)Mn_(0.33)Co_(0.33)O₂), and 2376 Wh/L(LiNi_(0.8)Co_(0.15)Al_(0.05)O₂), respectively. Accordingly, by usingthe nickel-based lithium-nickel composite oxide particles as apositive-electrode active substance of a lithium-ion battery, a batteryhaving high capacity can be prepared.

[Method for Producing Coated Lithium-nickel Composite Oxide Particles]

Various methods can be used as a method for producing coatedlithium-nickel composite oxide particles, that is, a method for coatingnickel-based lithium-nickel composite oxide particles with a conductivepolymer that becomes a shell.

For example, a method can be used for the production in which aconductive polymer is dissolved or dispersed in a solvent good for theconductive polymer, and particles are further mixed into the resultantmixture to prepare a slurry, and then a poor solvent is added to theconductive polymer and washed in a stepwise manner, the good solvent isthoroughly removed, and the conductive polymer is deposited on particlesurfaces, a so-called phase separation method.

Further, a method in which a conductive polymer that becomes a shell isdissolved or dispersed in a solvent good for the conductive polymer, andparticles that become cores are mixed into the resultant mixture toprepare a slurry, into this slurry, a solvent poor for the conductivepolymer is added and mixed,

and then the good solvent is gradually removed, and the conductivepolymer is precipitated on particle surfaces, a so-called interfacialprecipitation method, can also be used for the production.

In addition, a method in which particles that become cores are dispersedin a solution in which a conductive polymer has been dissolved ordispersed, and droplets are finely dispersed and sprayed in hot air, aso-called air drying method, or a spray drying method can also be usedfor the production.

Moreover, a method in which particles that become cores are allowed toflow by a rolling pan, to which a solution in which a conductive polymerhas been dissolved or dispersed is sprayed, and the particle surfacesare coated uniformly with the conductive polymer and dried, a so-calledpan coating method, can also be used for the production.

Furthermore, a method in which particles that become cores arecirculated up and down in a gas blown from the bottom, to which asolution in which a conductive polymer has been dissolved or dispersedis sprayed, so-called a gas suspension coating method, can also be usedfor the production.

Among them, from the viewpoint of the production cost, theabove-described phase separation method can be most preferably used forproduction.

EXAMPLES

Hereinafter, Examples of the present invention will be specificallydescribed with Comparative Examples. However, the present inventionshould not be limited to the following Examples.

Example 1

0.1 g of lignin graft type powder, polyaniline (emeraldine salt)manufactured by Sigma-Aldrich Co. LLC was dissolved in 284 g of ethanolto prepare a solution. As nickel-based lithium-nickel composite oxideparticles, 50 g of the composite oxide particles represented by thetransition metal composition Li_(1.03)Ni_(0.82)Co_(0.15)Al_(0.03) wasplaced into this solution, and 16 g of toluene was further added intothe resultant mixture and mixed to prepare a slurry. Next, the slurrywas transferred to an evaporator, the flask part was placed in a waterbath warmed to 45° C. under reduced pressure, and the ethanol wasremoved while rotating the flask. Subsequently, the preset temperatureof the water bath was set to 60° C., and the toluene was removed.Finally, in order to remove the solvent thoroughly, the powder wastransferred to a vacuum dryer, and dried at 100° C. for two hours underreduced pressure to prepare processed powder.

By using the particles coated with this polyaniline compound as thecoated lithium-nickel composite oxide particles according to Example 1,the following stability test in air, a gelation test, and a batterycharacteristics test (such as a charge and discharge test, and a cycletest) were performed.

Example 2

0.1 g of PEDOT/PSS (dry re-dispersible pellets) manufactured bySigma-Aldrich Co. LLC was dissolved in 284 g of ethanol to prepare asolution. As nickel-based lithium-nickel composite oxide particles, 50 gof the composite oxide particles represented by the transition metalcomposition of Li_(1.03)Ni_(0.82)Co_(0.15)Al_(0.03) was placed into thissolution, and into the resultant mixture, 16 g of toluene was furtheradded and mixed to prepare a slurry. The slurry was transferred to anevaporator, the flask part was placed in a water bath warmed to 45° C.under reduced pressure, and the ethanol was removed while rotating theflask. Subsequently, the preset temperature of the water bath was set to60° C., and the toluene was removed. Finally, in order to remove thesolvent thoroughly, the powder was transferred to a vacuum dryer, anddried at 100° C. for two hours under reduced pressure to prepareprocessed powder.

By using the particles coated with this PEDOT/PSS as the coatedlithium-nickel composite oxide particles according to Example 2, thefollowing stability test in air, a gelation test, and a batterycharacteristics test (such as a charge and discharge test, and a cycletest) were performed.

Comparative Example 1

The stability test in air, the gelation test, and the batterycharacteristics test were performed in the same manner as in Example 1and Example 2 except for using lithium-nickel composite oxide particlesthat had not been processed.

<Stability Test in Air>

2.0 g of lithium-nickel composite oxide particles according to each ofExample and Comparative Example was each put into a separate glassbottle, the glass bottles were left to stand in a thermostat at atemperature of 30° C. and humidity of 70% for one week, the increasedmass was measured as compared to the initial mass, and the change rateper particles mass was calculated. By setting the change rate perparticles mass of the lithium-nickel composite oxide particles after thelapse of one week according to Comparative Example 1 to 100, the changerate on every day of each of Examples 1 and 2 and Comparative Example 1was shown in FIG. 1.

As can be seen from FIG. 1, the coated lithium-nickel composite oxideparticles in Example 1, which had been coated with a polyanilinecompound, and the coated lithium-nickel composite oxide particles inExample 2, which had been coated with PEDOT/PSS, had a small change rateper mass as compared with that of the lithium-nickel composite oxideparticles in Comparative Example 1, which had not been coated with aconductive polymer. From this result, it was confirmed that by coatingthe particles with a polyaniline compound, or PEDOT/PSS, the permeationof moisture and carbon dioxide in the air can be suppressed.

<Gelation Test>

As to the measurement of change over time of the viscosity of thepositive electrode mixture slurry, a positive electrode mixture slurrywas prepared in the following order, and then the increase of viscosityand the gelation were observed.

As for the mixing ratio, lithium-nickel composite oxide particlesaccording to Examples and Comparative Example, a conductive auxiliary, abinder, N-methyl-2-pyrrolidone (NMP) were weighed so that the mass ratioof the lithium-nickel composite oxide particles:the conductiveauxiliary:the binder:the NMP was 45:2.5:2.5:50, further 1.5% by mass ofwater was added, then the resultant mixture was stirred by arotation-revolution mixer, and a positive electrode mixture slurry wasobtained. The obtained slurry was stored in an incubator at 25° C., andthe changes over time of the viscosity increase and the degree ofgelation in Examples 1 and 2 and Comparative Example 1 were confirmed,respectively, by stir mixing the slurry with a spatula. The slurry wasstored until obtaining complete gelation.

It took three days for the slurry according to Example 1 and Example 2to reach complete gelation, and it took one day for the slurry accordingto Comparative Example 1 to reach complete gelation. From this, in theslurry according to Example 1 and Example 2, by coating thelithium-nickel composite oxide particles with a polyaniline compound, orPEDOT/PSS, the generation of impurities such as lithium hydroxide (LiOH)and lithium carbonate (Li₂CO₃) was suppressed, and the slurry gelationand the slurry viscosity increase caused by the reaction with impuritiesand a binder can be prevented.

Further, in a case when the lithium-nickel composite oxide particleswere coated with a fluorine compound, the fluorine compound wasdissolved generally into N-methyl-2-pyrrolidone (NMP), therefore, it isconsidered that even though being coated with the fluorine compound, thecoated films are dissolved. Accordingly, different from the coatedlithium-nickel composite oxide particles according to Examples, it isconsidered to be difficult to suppress the generation of impurities whenthe produced positive electrode is stored. Therefore, the reaction withan electrolytic solution accompanied by gas generation in batterydriving, which is caused by the impurities generated during the storageof the positive electrode, is difficult to be suppressed, and anexpensive storage facility is required.

<Battery Characteristics Evaluation>

By the following procedures, a non-aqueous electrolyte secondary battery(lithium-ion secondary battery) for evaluation was prepared, and thebattery characteristics evaluation was performed.

[Production of Secondary Battery]

As for the battery characteristics evaluation of the lithium-nickelcomposite oxide particles of the present invention, a coin type batteryand a laminate type battery were prepared, and the coin type battery wassubjected to a charge and discharge capacity measurement and thelaminate cell type battery was subjected to a charge and discharge cycletest and a resistance measurement.

(a) Positive Electrode

Into the obtained lithium-nickel composite oxide particles according toExamples and Comparative Example, an acetylene black as a conductiveauxiliary, and polyvinylidene fluoride (PVdF) as a binder were mixed sothat the mass ratio of the particles, the acetylene black, and the PVdFwas 85:10:5, and the resultant mixture was dissolved into anN-methyl-2-pyrrolidone (NMP) solution to prepare a positive electrodemixture slurry. An aluminum foil was coated with the positive electrodemixture slurry by a comma coater and heated at 100° C. and dried, as aresult of which a positive electrode was obtained. A load was applied tothe obtained positive electrode through a roll press machine, and apositive electrode sheet in which the positive electrode density hadbeen increased was prepared. This positive electrode sheet was punchedout for the evaluation of the coin type battery so as to have thediameter of 9 mm, and also cut out for the evaluation of the laminatedcell type battery so as to have the size of 50 mm×30 mm, and each of thepunched-out sheet and the cut-out sheet was used as a positive electrodefor evaluation.

(b) Negative Electrode

Graphite as a negative electrode active substance and polyvinylidenefluoride (PVdF) as a binder were mixed so that the mass ratio of thegraphite and the PVdF was 92.5:7.3, and the resultant mixture wasdissolved into an N-methyl-2-pyrrolidone (NMP) solution to obtain anegative electrode mixture paste.

In the same manner as in the positive electrode, with this negativeelectrode mixture slurry, a copper foil was coated by a comma coater,and heated at 120° C. and dried, as a result of which a negativeelectrode was obtained. A load was applied to the obtained negativeelectrode through a roll press machine, and a negative electrode sheetin which the electrode density had been improved was prepared. Theobtained negative electrode sheet was punched out for the coin typebattery so as to have the diameter of 14 mm, and also cut out for thelaminated cell type battery so as to have the size of 54 mm×34 mm, andeach of the punched-out sheet and the cut-out sheet was used as anegative electrode for evaluation.

(c) Coin Battery and Laminated Cell Type Battery

The prepared electrode for evaluation was dried at 120° for 12 hours ina vacuum dryer. By using this positive electrode, a 2032 type coinbattery and a laminated cell type battery were prepared in a glove boxin which the dew point was controlled at −80° C. in an argon atmosphere.For the electrolytic solution, ethylene carbonate (EC) using 1M of LiPF₆as a supporting electrolyte and diethyl carbonate (DEC) (manufactured byTOMIYAMA PURE CHEMICAL INDUSTRIES, LTD.), the ratio of which was 3:7,were used, and a glass separator was used as a separator, to prepareeach of the batteries for evaluation.

<<Charge and Discharge Test>>

The prepared coin type battery was left to stand for around 24 hoursafter the assembly, and charged at a current density of 0.2 C rate up toa cut-off voltage of 4.3 V in a thermostat at 25° after the open circuitvoltage (OCV) was stabilized. After one hour of rest, a charge anddischarge test for measuring the discharge capacity was performed whenthe battery was discharged up to a cut-off voltage of 3.0 V.

The initial discharge capacity of the coin type battery according toExample 1 was 198.99 mAh/g, and the initial discharge capacity of thecoin type battery according to Example 2 was 191.91 mAh/g, but theinitial discharge capacity of the coin type battery according toComparative Example 1 was 191.93 mAh/g.

<<Cycle Test>>

In the same manner as in the coin type battery, the prepared laminatetype battery was left to stand for around 24 hours after the assembly,and charged at a current density of 0.2 C rate up to a cut-off voltageof 4.1 V in a thermostat at 25° C. after the open circuit voltage wasstabilized. After one hour of rest, the battery was discharged up to acut-off voltage of 3.0 V. Next, this battery was subjected to a cycletest of repeating a cycle of 4.1 V-CC charge and 3.0 V-CC discharge at acurrent density of 2.0 C rate in a thermostat at 60° C., and a cycletest of confirming, the capacity retention rate after 500 cycles wasperformed. The results of the cycle test are shown in FIG. 2, theimpedance test results before the cycle test are shown in FIG. 3, andthe impedance test results after the cycle test of 500 cycles are shownin FIG. 4.

From FIGS. 2 and 3, in the capacity retention before the cycle test andthe Cole-Cole plot in impedance, the laminate batteries according toExamples and Comparative Example were approximately equal to each other,but from FIGS. 2 and. 4, in the capacity retention after the impedancetest. after a cycle test of 500 cycles, the capacity retention of thelaminate type battery according to Example 1 and Example 2 was retainedhigher than that of the laminate type battery according to ComparativeExample 1. From this, it was confirmed that because the lithium-nickelcomposite oxide particles used for the laminate battery of Example iscoated with a polyaniline compound, or PEDOT/PSS, the decreased amountof the capacity retention is small also when used in a long cycle,therefore, the lithium-nickel composite oxide particles have highercapacity retention rate and are excellent.

1. Coated lithium-nickel composite oxide particles for a lithium-ionbattery positive-electrode active substance, comprising: nickel-basedlithium-nickel composite oxide particles, surfaces of which are coatedwith a conductive polymer.
 2. The coated lithium-nickel composite oxideparticles according to claim 1, wherein a coating amount of theconductive polymer is from 0.1 to 5.0% by mass based on 100% by mass ofthe lithium-nickel composite oxide particles.
 3. The coatedlithium-nickel composite oxide particles according to claim 1, whereinthe conductive polymer is a polymer or copolymer including at least oneselected from the group consisting of polypyrrole, polyaniline,polythiophene, poly(p-phenylene), polyfluorene, and a derivativethereof.
 4. The coated lithium-nickel composite oxide particlesaccording to claim 1, wherein the lithium-nickel composite oxide isrepresented by the following Formula (1),Li_(x)Ni_((l-y-z))M_(y)N_(z)O₂   (1) wherein x is a value of from 0.80to 1.10, y is a value of from 0.01 to 0.20, z is a value of from 0.01 to0.15, and l-y-z is a value exceeding 0.65, and M represents at least oneelement selected from Co or Mn, and N represents at least one elementselected from Al, In or Sn.
 5. The coated lithium-nickel composite oxideparticles according to claim 1, wherein the coated lithium-nickelcomposite oxide particles are spherical particles having an averageparticle diameter of from 5 to 20 μm.
 6. A method for producing thecoated lithium-nickel composite oxide particles according to claim 1,comprising: preparing a resin solution for coating by dissolving aconductive polymer into a good solvent that dissolves the conductivepolymer; adding a poor solvent that does not dissolve a resin forcoating and has a boiling point higher than that of the good solventinto the resin solution for coating; adding the lithium-nickel compositeoxide particles into the resin solution for coating to prepare a slurry;and removing the good solvent and the poor solvent sequentially from theslurry.